1965 Valiant low brake pressure

Not necessarily. You said it was fine for a while and then went all to hell. If everything else is right....which is tough to tell because of the now chinese wheel cylinders, the master cylinder is the only culprit. This is exactly why I say rebuild rebuild rebuild. That way, you know you have the correct cylinders. Only a couple of times have i ever not been able to rebuild a master of wheel cylinder or caliper. Plus, it's way more cost effective.